Isoolefin polymers are prepared in carbocationic polymerization processes. Of special importance is butyl rubber which is a copolymer of isobutylene with a small amount of isoprene. Butyl rubber is made by low temperature cationic polymerization that generally requires that the isobutylene have a purity of >99.5 wt % and the isoprene have a purity of >98.0 wt % to prepare high molecular weight butyl rubber.
The carbocationic polymerization of isobutylene and its copolymerization with comonomers like isoprene is mechanistically complex. See, e.g., Organic Chemistry, SIXTH EDITION, Morrison and Boyd, Prentice-Hall, 1084-1085, Englewood Cliffs, New Jersey 1992, and K. Matyjaszewski, ed, Cationic Polymerizations, Marcel Dekker, Inc., New York, 1996. The catalyst system is typically composed of two components: an initiator and a Lewis acid. Examples of Lewis acids include AlCl3 and BF3. Examples of initiators include Brønsted acids such as HCl, RCOOH (wherein R is an alkyl group), and H2O. During the polymerization process, in what is generally referred to as the initiation step, isobutylene reacts with the Lewis acid/initiator pair to produce a carbenium ion. Following, additional monomer units add to the formed carbenium ion in what is generally called the propagation step. These steps typically take place in a diluent or solvent. Temperature, diluent polarity, and counterions affect the chemistry of propagation. Of these, the diluent is typically considered important.
Industry has generally accepted widespread use of a slurry polymerization process to produce butyl rubber, polyisobutylene, etc. Typically, the polymerization process extensively uses methyl chloride at low temperatures, generally lower than −90° C., as the diluent for the reaction mixture. Methyl chloride is employed for a variety of reasons, including that it dissolves the monomers and aluminum chloride catalyst but not the polymer product. Methyl chloride also has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers. The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of approximately 26% to 37% by volume in the reaction mixture can be achieved, as opposed to the concentration of only about 8% to 12% in solution polymerization. An acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by surface heat exchange.
More recently, the polymerization of isobutylene and other monomers in hydrofluorocarbon (HFC) diluents has been disclosed. The utilization of HFC's in diluents or blends of diluents has created new polymerization systems that reduce particle agglomeration, and also can eliminate or reduce the amount of chlorinated hydrocarbons such as methyl chloride in polymerization systems. Such new polymerization systems reduce particle agglomeration and reactor fouling without having to compromise process parameters, conditions, or components and/or without sacrificing productivity/throughput and/or the ability to produce high molecular weight polymers. HFC's are chemicals that are currently used as environmentally friendly refrigerants because they have a very low (even zero) ozone depletion potential. Their low ozone depletion potential is thought to be related to the lack of chlorine. The HFC's also typically have low flammability particularly as compared to hydrocarbons and chlorinated hydrocarbons.
Some polymerization media, processes, reactors and systems that can employ HFC's are disclosed in the following commonly assigned patent references: WO2004058827; WO2004058828; WO2004058829; WO2004067577; WO2006011868; US2005101751; US2005107536; US2006079655; US2006084770; US2006094847; US2006100398; and US2006111522.
The use of HFC's in polymerization processes has also required finding new post-polymerization or “downstream” processes that can accommodate such new technology. For example, commonly assigned WO2006009550 discloses filtration to remove polymer from a slurry in an HFC-containing diluent. In addition, the diluent may contain components that need to be removed before the reactor effluent may be recycled to the polymerization process. Post polymerization reactor effluents containing isobutylene or other monomers are not usable as carriers for the catalyst system due to the polymerization of contained isobutylene before entry to the reactor and to the deleterious effects this has on catalyst system quality. Thus, it is essential to have a method for recovering a diluent such as HFC or at least a portion of the HFC from the post-polymerization reactor effluent before it may be recycled as a diluent into the polymerization process.
In conventional butyl rubber polymerization, isobutylene and methyl chloride can be easily separated by conventional distillation. However, azeotropic mixtures or azeotrope-like mixtures involving HFC's in other areas have been encountered in the past. See, e.g., U.S. Pat. Nos. 5,087,329, 5,200,431, 5,470,442, 5,723,429, 5,744,662, 5,830,325, 6,156,161, 6,307,115, 6,527,917, and EP 1 003 699 B. Some HFC's such as, for example, 1,1,1,2-tetrafluoroethane (“R134a”) and 1,1-difluoroethane (“R152a”) form maximum boiling azeotropes or azeotrope-like mixtures with isobutylene. Thus, the post-polymerization separation of certain HFC's from unreacted monomers such as isobutylene by simple distillation is not always possible. An extractive distillation method for separating slurry components from a polymerization reactor employing HFC diluent is disclosed in commonly assigned WO2006/009553.
The polydispersity, also called molecular weight distribution (MWD=Mw/Mn), of butyl rubber prepared commercially is typically broader than the expected most-probable distribution (2.0) and varies from plant to plant. For example, isobutylene polymerization in an HFC-containing diluent has been found to generally produce polymers with a narrower MWD than methyl chloride diluent.
It has now been discovered that the types and levels of trace impurities in the recycled diluents and unreacted monomers can be different for different production facilities, which may result from differences in the manner in which the various diluents and monomers are processed and recovered from the reactor effluents for recycle. For example, methyl chloride may hydrolyze when water is present, such as in water quenching of the reactor effluent in some production facilities, to form methanol and dimethyl ether, which can then contaminate recycled diluent and/or monomer if it is not removed in the recovery of the diluent and unreacted monomer. Moreover, fresh monomer and diluent makeup streams sometimes can contain relatively high contaminant levels that may go unnoticed. It is thus seen that the types and levels of impurities can fluctuate uncontrollably in a production facility. Steps are frequently undertaken to remove water and hydrolysis products from entering monomer and other feed streams to the reactor.
Heretofore, it has been common practice in the production of butyl rubber to adjust the rate of catalyst supply (Lewis acid and initiator) to the reactor to try to obtain the desired molecular weight of the polymer. In a typical production facility, the recovered polymer or polymer cement is sampled downstream from the reactor, and the molecular weight is gauged by Mooney viscometer readings or other rheological correlations, or by more rigorous testing such as gel permeation chromatography (GPC). If the molecular weight is off target, the catalyst feed has typically been adjusted, the process allowed to line out at steady state, and another reading taken to see if the molecular weight is closer to the target. The time between sampling and molecular weight determination is frequently as much as an hour or more, during which time off-spec product may be produced and other process conditions may have changed. Sometimes it can be difficult or seemingly impossible for one facility to produce a butyl rubber matching the specifications of another facility.
Chinese Patent Application No. 01134710.4, Public Disclosure No. CN 1417234A, discloses a method for the preparation of isoolefin polymers or copolymers by cationic polymerization in which a homopolymerization reaction of C4-C7 isoolefin monomers or a copolymerization reaction with other monomers is performed in a chlorohydrocarbon diluent using a Lewis acid as the primer, to which reaction system it is suggested to add such dispersing agents as carboxylic acid esters, ethers, ketones, amines, styrenes or alkyl substituted styrenes. The dispersing aids are said to lower the viscosity of the polymerization system and to make the dispersion of the insoluble polymer granules more uniform in the diluent. The reference claims that at a reaction temperature below −20° C., a stably dispersed polymer system can be obtained, the problem of heat transfer and mass transfer can be effectively improved, the dispersing agent that has been added can be easily obtained, and, at the same time, a narrower MWD of the polymer is obtained. However, there is no disclosure of any specific co-initiator for the Lewis acid, and some of the dispersing aids are known comonomers.
Applicant proposes herein that some types and levels of adventitious components in some polymerization reactor feeds may have a heretofore unrecognized and/or unpredictable effect on the molecular weight and/or MWD. Polymerization methods and systems to adjust the MWD in a controllable manner would be desirable for several reasons such as being able to more closely match the distribution of products prepared at different plants and/or with different diluents or catalyst systems, to tailor the distribution of products prepared in new processes, and potentially to prepare new product grades.